Temperature compensation for cesium beam frequency standard cavity



Nov. 8, 1966 A 3,284,720

TEMPERATURE (EDMPENSATION FOR CESIUM BEAM FREQUENCY STANDARD CAVITY Filed Feb. 17, 1964 CESIUM '13 15 BEAM F I--- DETE0T0R A Q P SOURCE I t I 25 tRIS ll 23 SIGNAL V SOURCE Fxze S|GNAL INPUT INVENTOR ALEXANDER BRANDLI BY Q-C,

AGENT United States Patent 3,284,720 TEMPERATURE COMPENSATION FOR CESIUM BEAM FREQUENCY STANDARD CAVITY Alexander Brandii, Palo Alto, Calif., assignor to Hewlett- Packard Company, Palo Alto, Calif., a corporation of California Filed Feb. 17, 1964, Ser. No. 345,343 2 Claims. (Cl. 331-3) This invention relates to temperature compensating apparatus for the microwave cavity of a cesium-beam frequency standard.

Cesium-beam frequency standards generally include apparatus for producing a molecular beam of neutral cesium (C atoms which are acted upon by a microwave signal having a frequency substantially equal to the atomic resonance of cesium. This produces a transition in the atomic structure of the cesium atoms in the beam which is detectable at the terminal end of the beam. One method of exciting the transitions in the cesium atoms is to pass the beam through apertures in a microwave resonant cavity, as shown in Comparison and Evaluation of Cesium Atomic Beam Frequency Standards, Holloway et al., Proceedings of the IRE, vol. 47, No. 10, page 1732, FIGURE 3. It is essential, however, that the resonant frequency of the cavity remain substantially constant.

Accordingly it is an object of the present invention to provide a temperature compensated resonator for a molecular beam frequency standard.

In accordance with the illustrated embodiment of the present invention, a cesium beam passes through apertures in a microwave cavity which has a resonant frequency that is maintained substantially constant with temperature by a cavity iris which includes a bimetallic strip.

Other and incidental objects of the present invention will be apparent from a reading of this specification and an inspection of the accompanying drawing in which:

FIGURE 1 is a pictorial diagram of the cesium beam frequency standard; and

FIGURE 2 is a sectional view of the iris of the present invention.

In the drawing, FIGURE 1 shows a cesium beam 9 which emanates from a source 11 and passes through apertures 13 and 15 in the microwave cavity 17 to the detector 19. A weak magnetic field 20 of the order of 50 m-illigauss exists in the region 22 about the two apertures. Utilization circuit 21 connected to detector 19 responds to a peak which appears in the beam current at the frequency of signal from source 23 which is equal to the atomic resonant frequency of cesium. The microwave structure 17 forms a resonant cavity having negligible relative phase shift between the microwave electromagnetic fields at the apertures 13 and 15. Microwave energy is applied to this cavity from signal source 23 through the temperature compensated iris 25, shown in FIGURE 2.

The iris 25 of FIGURE 2 includes a bimetallic conductive member 27 disposed normal to the broad walls of the waveguide 29 and transverse to the direction of propagation of microwave signal through the waveguide. This member 27 forms a boundary wall for the cavity 17 of FIGURE 1. Thus, the position of the member 27 which varies with temperature determines the resonant frequency of the cavity 17. In practice, the column 31, ring 35 and boss 33 are slidable along a slotted aperture 3,284,720 Patented Nov. 8, 1966 ICC 39 to perm-it initial tuning of the cavity. Member 27 is mounted in block 28 which is slidable in the column 31 to provide an initial adjustment. The block 28 is positioned in column 31 at a location therein for which the flexure of member 27 with temperature maintains the resonant frequency constant. The combination of the boss 33 on waveguide 29, the ring 35 and column 31 form a series of radial and coaxial lines 37 which present a short circuit to microwave signals appearing at the aperture 39 in the waveguide 29. Adjustment screw 41 protrudes through the broad wall of waveguide 29 at a point which is opposite and in line with aperture 39. The extent to which screw 41 protrudes into the guide beneath the lower tip 43 of member 27 determines the impedance presented to signal incident upon the iris. The screw 41 is adjusted to match the cavity to the characteristic impedance of waveguide 29.

I claim:

1. A cesium-beam frequency standard comprising:

a microwave cavity resonator having a microwave energy input part and an aperture therein;

means producing a cesium beam passing through said aperture in energy coupling relationship to the microwave signal in said cavity;

an iris disposed at said input part of the cavity to alter the resonant frequency thereof in response to the position of said iris;

said iris including a conductive member which responds to changes in temperature to alter position from a normal position relative to the cavity resonator for maintaining the resonant frequency of said cavity constant with temperature; and

means for coupling microwave signal through said iris into said cavity.

2. A cesium-beam frequency standard comprising:

a microwave cavity resonator including a U-shaped portion having apertures near the ends of the U- shaped portion;

means producing a cesium beam passing through said apertures in energy coupling relationship to the microwave signal in said cavity;

another portion of said cavity attached to said U- shaped portion at a point thereon intermediate the ends thereof;

an iris in said other portion of the cavity for supplying microwave signal therethrough;

said iris including a bimetallic conductive member which protrudes into said other portion of said cavity through a broad wall thereof and which responds to changes in temperature to alter position in a selected direction from a normal position relative to the cavity resonator for maintaining the resonant frequency of said cavity substantially constant with temperature;

said conductive member being aligned with the selected direction parallel to the direction of propagation of microwave signal into said other portion of the cavity.

References Cited by the Examiner UNITED STATES PATENTS 3,108,240 10/1963 Riblet 333-83 3,209,200 9/1965 Nelson et al 33383 NATHAN KAUFMAN, Primary Examiner.

D. R. HOSTETTER, Assistant Examiner. 

1. A CESIUM-BEAM FREQUENCY STANDARD COMPRISING: A MICROWAVE CAVITY RESONATOR HAVING A MICROWAVE ENERGY INPUT PART AND AN APERTURE THEREIN; MEANS PRODUCING A CESIUM BEAM PASSING THROUGH SAID APERTURE IN ENERGY COUPLING RELATIONSHIP TO THE MICROWAVE SIGNAL IN SAID CAVITY; AN IRIS DISPOSED AT SAID INPUT PART OF THE CAVITY TO ALTER THE RESONANT FREQUENCY THEREOF IN RESPONSE TO THE POSITION OF SAID IRIS; SAID IRIS INCLUDING A CONDUCTIVE MEMBER WHICH RESPONDS TO CHANGES IN TEMPERATURE TO ALTER POSITION FROM A NORMAL POSITION RELATIVE TO THE CAVITY RESONATOR FOR MAINTAINING THE RESONANT FREQUENCY OF SAID CAVITY CONSTANT WITH TEMPERATURE; AND MEANS FOR COUPLING MICROWAVE SIGNAL THROUGH SAID IRIS INTO SAID CAVITY. 